The purpose of this study is to examine the myogenic mechanism of blood flow regulation in the microcirculation. To carry out this study we will measure intravascular pressure, red cell velocity and vessel diameter in arterioles of cat mesentery and cat sartorius muscle. Arterial pressure and venous pressure to the preparation will be changed in a stepwise fashion while recording the above three variables. In addition, we will do graded micro-occlusions of the arterioles while measuring these three factors. By comparing the vessel diameter at different levels of flow and intravascular pressure we expect to determine the relative influence of metabolic and myogenic factors on vessel diameter. We also plan to do similar studies on isolated preparations of small arteries which we have shown possess autoregulatory activity in situ in the sartorius muscle preparation.